In a device of this type, known from DE 20 35 584 A1, the flattening plates are secured to an auxiliary frame which is mounted for turning to a primary frame by means of a transverse element and a supporting axle. Mounted laterally in the supporting axle and swivelable are a first turning bar with its mounting and sleeve, the second deflecting roller with its mounting and sleeve, and a second turning bar with its mounting and sleeve. The deflecting rollers and turning bars are mounted in the supporting axle in reversible, turning manner. Since the turning bars must be positioned close to the vertical supporting axle and the distance of the turning bar from the supporting axle can only be modified within narrow limits, the diameter of the turning bars can also only be modified within narrow limits; thus the diameter of the turning bars cannot be increased at will, which may nonetheless be desirable. In the known device, therefore, not only is the diameter of the turning bars restricted, the turning bars are also positioned very close to the vertical axle that supports them, and this makes it difficult to guide the film webs around the turning bars when the device is installed.